In recent years, nanotechnologies have been gaining attention, and aim to produce different properties from those on a bulk or molecular level by making substances of nanometer size. Meanwhile, the application of recyclable natural fibers has also received increasing attention for substitution of petroleum resources and due to increasing environmental consciousness.
Among natural fibers, a cellulose fiber, particularly, a tree-derived cellulose fiber (pulp), is widely used mainly as a paper product. The width of the cellulose fiber that is used for paper is mostly 10 μm to 50 μm. Paper (sheets) obtained from the cellulose fiber is opaque, and, due to its opaqueness, is widely used as printing paper. Meanwhile, a cellulose fiber is treated (beaten or crushed) using a refiner or kneader, a sand grinder, and the like, and micronized (micro-fibrillation), thereby producing transparent paper (glassine paper or the like). However, the transparency of the transparent paper is on a semi-transparent level, the permeability of light rays is poor compared with a polymer film, and the degree of cloudiness (base value) is also large.
In addition, since the cellulose fiber is a collection of cellulose crystals having a high modulus of elasticity and a low coefficient of thermal expansion, and the thermal-resistant dimensional stability is improved by making the cellulose fiber into a composite with a polymer, the cellulose fiber is used for a laminate and the like. However, since the ordinary cellulose fiber is a collection of crystals, and has a tubular space therein, the cellulose fiber has a limitation with regard to the dimensional stability.
A water dispersion of microfibrous cellulose that is produced by mechanically crushing a cellulose fiber so as to be 50 nm or less in the fiber width is transparent. Meanwhile, while a microfibrous cellulose sheet includes spaces, irregularly reflects light rays so as to look white, and has an increasing opaqueness, when a resin is impregnated in the microfibrous cellulose sheet, the spaces are filled, and therefore a transparent sheet can be obtained. Furthermore, since the fibers of the microfibrous cellulose sheet are a collection of cellulose crystals, are extremely rigid, and have a small width, the number of fibers is markedly increased in the same mass compared with an ordinary cellulose sheet (paper). Therefore, when the microfibrous cellulose is made into a composite with a polymer, microfibers are more evenly and densely dispersed in the polymer, and the thermal-resistant dimensional stability is extremely improved. In addition, the fibers are fine, and thus the transparency is high. There are extremely high expectations for the composite of the microfibrous cellulose having the above characteristics to act as a flexible transparent substrate (a transparent substrate that can be bent and folded) for organic ELs and liquid crystal displays.
A number of techniques for micronizing microfibrous cellulose and techniques for making microfibrous cellulose into a composite with a polymer are disclosed, but only few techniques are disclosed in which microfibrous cellulose is made into a composite sheet while maintaining the industrial productivity.
Specifically, Patent Documents 1 to 3 disclose techniques for making a cellulose fiber into a microfibrous fiber, but these documents do not disclose or propose techniques for making the microfibrous cellulose into a sheet and, at the same time, making the microfibrous cellulose into a composite with a polymer.
Patent Documents 4 to 10 disclose techniques and the like in which properties, such as dynamic strength, are improved by forming a composite of microfibrous cellulose in a polymer resin, but disclose only few techniques for easing the formation of the composite.
In addition, Patent Documents 10 to 20 disclose techniques for making microfibrous cellulose into a sheet, but fail to secure the productivity on an industrial level, and thus there is a demand for provision of a simple method for making microfibrous cellulose into a composite sheet in a complex with a polymer, and a simple method for laminating the composite sheets.